Absorption cell based on rectangular prism is used in atmospheric monitor sensor with optical fiber. Times of probing beams propagating back and forth in the cell is determined by distance between symmetry planes through respective apexes of the prisms. The sensitivity of sensor is set by means of adjusting the distance. Taking difference of absorbance for output, the distance is adjusted at different amount of absorbance. The distance is set automatically by step motor under a mono-computer control system, and the receiver of output light beams is moved at corresponding place. An achromatic lens is taken for receiving lens, so that error of adjusting the distance can't affect the setting of sensitivity obviously.